Recently, oil pans are generally made of thin light alloy cast to shape for the purpose of lightening an engine with its associated elements. One such oil pan is disclosed in, for example, Japanese Utility Model Unexamined Publication No. 61-173,754 entitled "Oil Pan", laid open Oct. 29, 1986.
Such an oil pan, which is attached to the bottom of an engine block with cap screws or bolts at several points, has a circular arcuate flange integrally formed on one side surface thereof to which a transmission case is attached with cap screws or bolts. In such an oil pan, for fixing the oil pan having the circular arcuate flange to the bottom of the engine block with cap screws or bolts, the circular arcuate flange is formed with bosses through which the bolts are screwed in the bottom of the engine block. Because the boss is formed spaced apart from the fitting surface of the oil pan to the engine block, the boss is weak in rigidity and is liable to be deformed when the bolts are tightly screwed in the bottom of the engine block. This makes it hard to tightly fasten the oil pan to the engine block. Furthermore, because the circular arcuate flange has an downward extension below the bottom of the oil pan, the flange is also liable to be deformed by vibrations of a transmission case attached thereto.